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                [Usaco2006 Oct]Building the Moat护城河的挖掘                </h1>
                <p>时间限制：3s&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;  空间限制：64MB</p>			</div>
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                                <h3>题目描述</h3><p><p>为了防止口渴的食蚁兽进入他的农场，Farmer John决定在他的农场周围挖一条护城河。农场里一共有N(8&lt;=N&lt;=5,000)股泉水，并且，护城河总是笔直地连接在河道上的相邻的两股泉水。护城河必须能保护所有的泉水，也就是说，能包围所有的泉水。泉水一定在护城河的内部，或者恰好在河道上。当然，护城河构成一个封闭的环。 挖护城河是一项昂贵的工程，于是，节约的FJ希望护城河的总长度尽量小。请你写个程序计算一下，在满足需求的条件下，护城河的总长最小是多少。 所有泉水的坐标都在范围为(1..10,000,000,1..10,000,000)的整点上，一股泉水对应着一个唯一确定的坐标。并且，任意三股泉水都不在一条直线上。 以下是一幅包含20股泉水的地图，泉水用&quot;*&quot;表示</p>
<p><img height="387" width="325" alt="" src="../file/1670_0.jpg" /><br />
图中的直线，为护城河的最优挖掘方案，即能围住所有泉水的最短路线。 路线从左上角起，经过泉水的坐标依次是：(18,0),(6,-6),(0,-5),(-3,-3),(-17,0),(-7,7),(0,4),(3,3)。绕行一周的路径总长为70.8700576850888(...)。答案只需要保留两位小数，于是输出是70.87。</p>
<p></p></p><hr/><h3>输入格式</h3><p><p><span style="font-size: medium">* 第1行: 一个整数，N * 第2..N+1行: 每行包含2个用空格隔开的整数，x[i]和y[i]，即第i股泉水的位 置坐标 </span></p></p><hr/><h3>输出格式</h3><p><p><span style="font-size: medium">* 第1行: 输出一个数字，表示满足条件的护城河的最短长度。保留两位小数 </span></p></p><hr/><h3>样例输入</h3><pre>20
2 10
3 7
22 15
12 11
20 3
28 9
1 12
9 3
14 14
25 6
8 1
25 1
28 4
24 12
4 15
13 5
26 5
21 11
24 4
1 8
</pre><hr/><h3>样例输出</h3><pre>70.87</pre><hr/><h3>提示</h3><p>没有写明提示</p><hr/><h3>题目来源</h3><p>凸包 卡壳</p>
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